Even Windows XP is too advanced for some banks
About 6 or 7 years ago now, here in darkest Southern California, I was waiting in line for an ATM from a major American retail bank chain. The person in front of me appeared to be having problems and walked away in disgust. As they did so I could see the ATM had crashed and was rebooting.
At the time, never having worked in banking, I had naively assumed a modern ATM would just be a thin terminal of some kind with a custom hardware link to the cash dispensing machinery. However not only could I see this was a regular PC from the BIOS POST but that the OS it was booting was not any version of Windows at all, XP or otherwise. It was running IBM OS/2. It was not even OS/2 Warp!
This floored me for a minute until I understood the sheer brilliance of this. Whilst I don't doubt there are plenty of vulnerabilities in this dinosaur oddity of an OS where would go to you get hacking tools for it? Could you even setup a OS/2 VM to test against it on a modern hypervisor?